Suitable resin chemistries

For a UV-curable resin to be suitable for 3D printing i guess the critical property must be how photosensitive it is, to maximize print speed. I'm a chemist by profession but i haven't any experience with polymer chemistry, and i think i'd like to read up on the subject. What mechanism(s) cause a material to photopolymerise, and how would one optimize this behaviour? I've read some very interesting hints about what might be done, like cooking a UV-resin from linseed oil (Green Chem, 2011, 13, 1014 DOI: 10.1039/c1gc15038c). With a bit of research, it sounds to me like it would be possible to make your own resin from mostly renewable, dirt-cheap or even self-produced precursors. I also read somewhere about someone who built a kayak with epoxy resin synthesized from, i think it was, linseed oil, but i can't find any details on this. Epoxy is a bit pricey, it'd be awsum to be able to DIY in my opinion. Anyone else here thinking in these patterns? Is there any opensource development being done in this area?

I haven't decided anything yet but it seems to me that laser is the way to go. However, i'll need some kind of instruction to follow for the build - laser optics, circuit design and programming are not my areas of expertise.

I can think of a number of reasons for choosing the laser-based design:
* Cost. I have a lot of time, but correspondingly little (no) money.
* Intensity.
* X,Y Resolution. With the projector resolution is limited by the resolution of the projector, with the laser it's the lasers wavelength that decides..need to say more?

The only reason i can see for going with the projector is that it's probably simpler, maybe alot.

Polymerization speed is generally determined by the following
Light intensity: Increasing the intensity increases the polymerization rate. The numbers I have seen are far from perfect but it seems to be somewhere between I=Rp and I=Rp^2
Functionality of the monomers and oligomers: More possible reaction sites increases the polymerization rate, but also increases shrinkage and heat generation while decreasing the overall conversion of the C=C bonds and generally changing the vast majority of the cured properties. With some exceptions monofunctional monomers tend toward adhesives while trifunctional or greater tends toward hardcoats.
Photoinitiator chemistry: The most common initiators absorb a photon to generate a free radical which initiates the polymerization, the more efficient they are at doing so the faster the reaction. There are also photoinitiators which will generate multiple radicals or oligomeric initiators but they tend to be more expensive.

Direct use of epoxy polymers may be possible but the chemistry is different. They tend to be significantly slower than acrylate systems (though that does not mean much when acrylates coated at 1/2 mil thickness can be polymerized in fractions of a second) and are frequently used because they exhibit dark cure, meaning that if a portion of the part does not get enough light it will still cure. UV epoxies, known as cationic UV systems, are also generally heated at the end of polymerization to help drive it to completion.

As far as the materials go, there are already several companies producing resins which are at least partially biobased and more products frequently coming out. Acrylated epoxies are an option but tend to be incredibly viscous at room temperature. DIY production of the materials is unlikely as some of the materials have the tendency, if not handled properly, to polymerize... explosively.

... I've made some tests with UV-curing resins with a direct beamed+focussed 405nm-diodelaser (with 200mW max.) - with a spot of maybe 0.05mm and around 50mW of power the curing time was some ten microseconds per 'pixel', so maybe 2-5m/s max. possible moving speed with 0.01mm resolution.

With a 405nm-LED with 9Watts of power beaming across a DLP-chip the curing for the complete exposed area was some milliseconds, so much faster per layer than 'drawing' vector-lines.

But actually I'm more in testing+developing SLS/SLM with different materials like plastics, glass/ceramics and metals ...

Can you provide any links/documentation that advise on what type, how much and how to add fillers to a UV cured resin.
Graphite powder is one type of filler that I'm interested in adding, but it would be neat if there were other types that could be added to change the performance properties of the resin.

QuoteAzrael511UV epoxies, known as cationic UV systems, are also generally heated at the end of polymerization to help drive it to completion.

3D Systems Stereolithography machines use epoxies, but, while there's a curing "oven", the process doesn't include intentional heating. This curing oven is just a box with many UV lamps inside and well vented to actually avoid warming.

@A2
In my professional experience I do not typically use fillers or pigments since the resins are used for a purpose which requires clarity. That being said, if the viscosity is reasonably low and you do not require very high concentration you can usually just stir in fillers. Getting them to remain stable in the resin can be more of a challenge and that is where I have no expertise. The fillers are going to change the response of the resin quite a bit so I would recommend just try it with a very small amount of resin and see what you get.

@Traumflug
I took a look at the 3D systems machines and resins and they are interesting. The actual resins look to be both epoxy based and hybrid epoxy/acrylate systems. This makes a lot of sense when you need highly accurate parts, as the epoxy resins tend to shrink and warp less than high functionality acrylates. In my quick search I did not see their oven but if it uses traditional UV sources even with the ventilation it will likely get quite hot.

I am not aware of any ongoing open source development for resins but I would like to do some work in that realm. I have several years of experience in UV resins but currently no access to a DLP or Stereolithography based printer.

Ok, a DLP printer would be much simpler, but would have a very limited build volume and also a definite limit to resolution. A galvo-based laser system, possibly with more than one laser, would greatly overcome these limitations, and - at least in theory - be even cheaper to build. Sounds like a challenge to me!

There's one thing i don't understand: whether you're working from top or bottom, there will be times when you print at a point with uncured resin below/on top of the desired "pixel". Beam intensity will decrease exponentially, won't this result in a varying region of undercured material? How is this problem overcome, do you rely on the UV absorbance of the resin being high enough that this region will be negligible?

Thanks for all the response btw, it's been invaluable already! Next week i'll go to the library and get a book on polymer chemistry, seems i've some reading to do...

Quotenastybyte
a DLP printer would be much simpler, but would have a very limited build volume and also a definite limit to resolution.

I've been looking into this more, and there is a new DLP-SLA technique where you can expose tiled portions of a much larger image with the same resolution 25microns X/Y.
[www.buildyourownsla.com]

DLP-SLA build size with a 1920x1080pixels is 192mm x 108mm (7.5 in x 4.3 in) at 0.10mm resolution, which is as good, and possibly maybe a little better than the Shapeways ultra detail DLP-SLA printer.
I think that's a pretty big part considering you can go over 14.0 inches in the Z-axis.

Quotenastybyte
There's one thing i don't understand: whether you're working from top or bottom, there will be times when you print at a point with uncured resin below/on top of the desired "pixel". Beam intensity will decrease exponentially, won't this result in a varying region of undercured material? How is this problem overcome, do you rely on the UV absorbance of the resin being high enough that this region will be negligible?

From what I have read the top-down produces a bulge on the perimeter, where as the bottom up does not.

Where there is an undercut support material is required, this is true for any printing process.
Initially the beam is focused onto the stage. Subsequent layers absorb penetrating light, where there is an undercut, support material is placed to absorb excess light.

There is another method that uses a LCD mask. One person said it can produce a more accurate part, but LCD screens will break eventually from the damaging UV light.

I've found a few examples of people using LED projectors with success, but without details, most of what I have read is saying LED projectors do not work.
So I think some time is required for others to develop this new technology, it looks promising, but I just cant find enough information on it.

I've updated my pro's and con's list DLP-SLA vs. STL:

Advantageous of Top down DLP-SLA:
1. Less failed prints because it eliminates all problems associated with the part sticking to the bottom of the vat.
2. Eliminates the problem of the build plate vacuum.

Cons of Top down DLP-SLA:
1. Bubbles float to the top print surface.
2. As you introduce more solid material (elevator) the resin level goes up which could
give a small difference on Z axis.
3. You need to have a full Z deep building area.
4. There is a slight bulge due to surface tension. Some have used a wiper to try and alleviate it.

Advantages of Bottom up DLP-SLA:
1. Bubbles are less of a problem as they tend to float up.
2. Resin level is easier to maintain.
3. A shallow vat can be used, which uses less resin.
4. The bulge that top down introduces is not an issue because there is not surface tension.

Cons of Bottom up DLP-SLA:
1. Failed prints occur when parts stick to the bottom of the vat.
2. Build plate vacuum is difficult to overcome.

Advantages of DLP-SLA:
1. Larger build size possible: By moving the projected image across the vat bottom you are able to expose tiled portions of a much larger image on a vat bottom at a resolution of 25microns X/Y.
2. Are mechanically simpler, so they're easier to maintain. This also means they're quieter.
3. Have higher print quality because they use HD projectors to flash the cross sections.
4. For the same number of z-steps they print faster because all parts of a cross section are flashed simultaneously. There is no traveling extruder.
5. DLP will be faster for large objects compared to Laser printers.
6. Because the prints don't need to cool they're not prone to warping.
7. Because they don't use extruders and the substrate is liquid they will never jam.

Disadvantages of DLP-SLA:
1. Limited by the resolution of the printer.
2. The bigger the volume, the less accurate the image.
3. The larger the print, the better peeling mechanism and resin you need.

Advantages STL (laser)
1. Lasers are accurate, small and low power.
2. Laser will be faster for small objects.
3. Laser outline a vector of the printed object for each build layer.

Disadvantages STL (laser)
1. Can be slow to cure a whole solid layer, and large object take longer to build than DLP-SLA.
2. Vibrations need to be isolated from the printing process or part geometry and surface finish will be effected.
3. Requires XY linear guides, i.e. more mechanics, and stepper motors required.

QuoteA2I've been looking into this more, and there is a new DLP-SLA technique where you can expose tiled portions of a much larger image with the same resolution 25microns X/Y.

Printing tiles requires an accurate X-Y-axis. Without tiles, you don't need these axes at all.

QuoteA2DLP-SLA build size with a 1920x1080pixels is 192mm x 108mm (7.5 in x 4.3 in) at 0.10mm resolution, which is as good, and possibly maybe a little better than the Shapeways ultra detail DLP-SLA printer.
I think that's a pretty big part considering you can go over 14.0 inches in the Z-axis.

Yes, reasonable big, but also a pretty poor resolution for SLA printers.

QuoteA2Disadvantages STL (laser)
1. Can be slow to cure a whole solid layer, and large object take longer to build than DLP-SLA.
2. Vibrations need to be isolated from the printing process or part geometry and surface finish will be effected.
3. Requires XY linear guides, i.e. more mechanics, and stepper motors required.

Using galvanometers, moving mass is around 1 gram turning around its center, so almost no mass and accordingly no vibrations and no linear guides required. On a 3D Systems SLA you don't notice the galvos moving, you just see the light point wandering around.

With DLP-SLA the closer you get the more pixels you have per mm, which equates to higher XY resolution.
I found a new DLP-SLA printer that has a feature of normal and high resolution printing. They are moving the DLP projector closer to increase the resolution.
So if you move half the distance closer (I don't know what the ratio is, just an example), you reduce your max print size by half, and I suspect that you increase the resolution by a factor of 2x.

I have to use a wax in a carrier monomer type liquid to form the chain bonds. known as
TRPGDA (Tripropylene glycol diacrylate) from SHAMROCK TECHNOLOGIES inc. in their “everglide range of products” UV-691 , UV-636 , UV-600D , UV-395D

I realise this thread is quite old, but thought I'd add some general information in case it's any use to anyone.

Polymers are long-chain molecules that form from simpler molecules. These simpler molecules are called monomers, a single unit in the polymer chain, or oligomers, which are a few monomers joined together into a short chain.

Monomers typically have a double bond between two carbon atoms (an unsaturated organic molecule). Polymerisation occurs by breaking this double bond to connect the molecule to two other monomers which have also had their double bond broken. It requires a fair amount of energy to break this bond and this energy can be supplied in a number of different ways, typically either chemically through interaction with a free radical (which supplies an extra electron to break the bond) or through the application of heat and pressure.

In most photopolymerisation processes, the monomer resin is mixed with a photoinitiator. This photoinitiator produces a free radical when exposed to UV light, and it is this free radical that actually begins the polymerisation.

Most photosensitive resins are actually just ordinary resins, such as epoxy, mixed with a photoinitiator such as benzophenone. There are many other possibilities. Most recipes also include a UV blocker (so that curing doesn't penetrate too deeply) and a reactive diluent to make the resin less viscous (run better).

To respond to a few of your other points:

The cost tradeoff between DLP and laser systems depends a lot on where you buy them. It's relatively easy to pick up a second-hand DLP projector, much harder to pick up a second-hand 1W 405nm laser. 500mW 405nm lasers can be had on ebay for under US$100, but they're not dirt cheap as such. Print speed for a laser is directly related to the power output of the laser and high-powered lasers get expensive (and dangerous!) fairly quickly. There are people advertising higher-powered blue/UV lasers on alibaba, but they are generally not saying how much. Often you only get the diode and you have to sort out cooling and optics for yourself.

The resolution of a laser system is not limited by the wavelength. It's limited by how well you can focus the laser beam to a point. This depends in turn on the beam divergence of the light as it exits the laser and the quality of any optics in between. If you're using a galvo to steer the laser (which you almost certainly are) then the quality of the mirrors matters. If you're using a sub-$100 laser off ebay (in fact just about any solid-state laser), your beam quality is probably not very good. If you're using a cheap lens, it's going to affect your resolution. These are all things where cheap can be had cheap but quality can start to really hurt your hip pocket. And let's face it; do you really need a printer with <1um resolution? Unlikely.

Cheap galvo heads can indeed be had on ebay. They will almost certainly have an ILDA interface which is intended for laser light shows. This requires that your controller can output +/- 10V control signal at whatever rate you want to scan the thing. This is not exactly difficult, but it's non-trivial too. A 192kHz stereo DAC intended for high-end audio applications with a couple of op-amps to scale it to the right voltage as a differential signal will probably do the job, but you'll have to get a circuit board made and so on. If you're into low-cost hacking your own, I'd suggest that the tutorials kicking around on how to create a galvo from a computer fan are a good way to go. Cheap fans can be had for about a dollar. The control will be open-loop, so you'll have to calibrate them carefully, but the control should be good and the control circuitry will be trivial - depending on the current draw, you can either hook them up directly to an arduino, or you might need a MOSFET in the middle to provide more current capability.

The last thing to figure out is how you're going to stop the print from sticking to the bottom of the vat. Making the vat bottom out of FEP film seems to be the best answer for DIY hackers at this stage, though it has issues with the bottom of the vat drooping if the build area is too large. It's also still possible for the print to stick to it and cause odd layering effects. Raising the build plate on one side first and then the other can help with this but that then cuts your print speed.

Others are experimenting with other methods. The Carbon printer works by making the bottom of the vat permeable to oxygen and making the bottom of the vat also the top of a pressurised, oxygen-filled chamber. This creates a layer of oxygen-rich resin at the bottom of the vat. Oxygen inhibits polymerisation, so you get the curing happening slightly above the bottom of the vat. This prevents issues with the print sticking to the vat, if you can control the thickness of the oxygen-rich layer accurately (they're not saying exactly how they do this).

Or the Peachy printer (aside from its embezzlement problems) shines the laser from the top and drips salt water into the resin. The resin floats on the water and so the resin level gradually rises. By controlling the flow rate of water accurately, the location of the top of the resin can be controlled. This of course requires either a laser beam collimated at the spot size you want or variable-focus optics.

... I've tested laserscanning with a cheap XY-galvoscanner (200€), a pretty good DAC (300€) and a [email protected] with optics (200€) - the biggest hurdle is the focus size and shape ... with the needed FL of >150mm its not a round spot, but more a line with 100x400 microns!

With a DLP-setup this was much simpler and cheaper - found different 'lampless' DLP-beamers from 10€ to 200€ and modified them all to run without lamp and inserted a [email protected] (35€) or a (watercooled) [email protected] (>300€).

The possible XY-resolutions are from 100 microns with the standard optics down to 10 microns with reducing lenses ... but then with reduced working area too ...